He says the amount of payment considered "reasonable and not an inducement" varies markedly from one institutional ethics committee to another.

Reimbursement of costs

Most researchers make a distinction between reimbursement of out-of-pocket costs and other payments.

Dr Martin Stockler, co-director of cancer trials at the University of Sydney's NHMRC Clinical Trials Centre, says that patients participating in clinical trials are seldom reimbursed and reimbursement may be more common in studies involving healthy volunteers.

Professor Phyllis Butow at the University of Sydney trains researchers in ethical ways to gain consent from participants.

She says most clinical researchers think reimbursement is fair and ethical because participants are contributing to community wellbeing.

"Reimbursement is always something that should be considered because people shouldn't be put out of pocket for giving in this way," she says.

But, says Butow, "probably some reimbursement is more generous than others".

"I guess it becomes a bit grey, especially if you start to think about compensating people for time, whether some people's time is worth more than other people's time."

Butow thinks clearer guidelines would be helpful if difficult to write because they will need to cover a huge diversity of research types. But she thinks large payments to participants should be ruled out in any new guidelines.

The NHMRC guidelines on ethics in human research are being revised but it is understood the guidance on paying study participants will not change.